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Close binaries of supermassive black holes (CB-SMBHs) with separations of$\lesssim 0.1$pc as the final stage of galaxy mergers are sources of lowfrequency gravitational waves (GW), however, they are still elusiveobservationally because they are not spatially resolved. Fortunately,reverberation as echoes of broad emission lines to ionizing continuum conveysinvaluable information of the dynamics of broad-line regions (BLRs) governed bysupermassive black holes in the central regions of active galactic nuclei(AGNs). In this paper, we demonstrate how to composite the hybrid 2-dimensionaltransfer functions of binary BLRs around the CB-SMBHs in AGNs, providing anopportunity of identifying them from reverberation mapping (RM) data. It isfound that there are variation-coupling effects in the transfer functions,arising from the coupling of CB-SMBH light curves in the Fourier space. Weprovide semi-analytical formulations of the transfer functions for kinematicmaps of the gas. For cases with the simplest variation-coupling effects, wemake calculations for several BLR models and reveal significant distinctionsfrom those of single active black holes. In principle, the difference is causedby the orbital motion of the CB-SMBH systems. In order to search for CB-SMBHsin time-domain space, selection of target candidates should focus on local AGNswith H$\beta$ double-peaked profiles and weaker near-infrared emission.High-fidelity RM-campaigns of monitoring the targets in future will provideopportunities to reveal these kinematic signatures of the CB-SMBHs and hencefor measurements of their orbital parameters.

Kinematic signatures of reverberation mapping of close binaries of supermassive black holes in active galactic nuclei